your comfort with him might have triggered that "any minute now" reaction in your body. generally, when we are attacked (sexually or otherwise) it's at a time when we are least expecting it...which is part of what makes it traumatic.

it's likely really difficult for you to relax and maybe trusting him and letting your guard down has triggered a self-protective reaction.

is there anything you might be able to do to feel more protected, so that you can start talking about whats going on? like, bring someone you trust with you to session or change the positioning so you feel like you could bolt from the room if you needed to?